Various methods of opalizing light bulbs have been proposed and, at the industrial level, two known process are used.
According to the process known as the "wet process", a suspension of titanium dioxide, carried in a liquid that is evaporated by bringing the bulb to a temperature of 400.degree. to 500.degree. C., is deposited on the inside of the light bulb.
According to another process known as the "dry process", a film of silica containing titanium dioxide is applied by spraying a powder on the inside of the light bulb.
Gaseous opalization methods are also known, but they are very few in number and do not appear to be adapted for use in industry. The formation of deposits of titanium dioxide by hydrolysis of titanium compounds, such as titanium tetrachloride, is taught in U.S. Pat. No. 3,352,703. According to this patented process, vapors of moist air and titanium tetrachloride are passed through an electric arc. The moist air is produced by bubbling air in water. At the start, the electric arc is generated at an alternating voltage of about 5,000 volts, after striking, the alternating voltage is maintained at about 1,100 volts. French patent application No. 2,304,169 relates to a process of forming a white light diffusible deposit on a wall of transparent glass by the combustion of a mixture of hydrogen, air and a vapor comprising at least an organic silicon compound. According to this technique, the particles coming from the combustion fumes of said gaseous mixture are directed and their distribution controlled, in terms of thickness, by means of an electric field that prevails around and in the bulb. According to German application 2906038 it is known how to form a deposit of silica and boron oxides from a gaseous mixture of SiCl.sub.4 and BCl.sub.2 O.sub.3, which is subjected to a pyrolysis or hydrolysis treatment. Alumina deposits can also be produced by the combustion of organic aluminum compounds, which are dissolved in a solvent, taught in U.S. Pat. Nos. 3,842,306 and 3,868,266. In U.S. Pat. No. 3,109,747 there is described the production of a silica deposit by the combustion of tetraethyl silicate by producing a process involving several relatively complex stages.